Discussion on Lightning Protection Measures for Electronic Weighbridge Weighing Sensors

1. Introduction

From 2005 to 2006, the Mineral Products Tax Collection Management Bureau of our city adhered to the principle of installing while preventing lightning, and designated our bureau to design and construct lightning protection projects for electronic weighing scales at 22 collection stations (due to the fact that all three electronic weighing scales installed by the bureau in early 2004 were struck by lightning during use, with Zhushi Station almost paralyzed). In the 14 electronic weighbridge lightning protection projects constructed by our bureau in 2005, due to insufficient understanding of electronic weighbridge weighing sensors and neglect of corresponding lightning protection measures, 6 stations were hit by lightning, damaging 9 weighing sensors and 2 weighing displays. In 2006, targetingLoad CellDue to the issue of being prone to lightning strikes, our bureau has revised the design and completed the construction of 8 other electronic weighbridge lightning protection projects. At the same time, we have organized technical forces to rectify the lightning protection projects installed in 2005. However, due to inadequate technical measures, 5 stations still suffered lightning strikes to varying degrees, damaging 6 sensors. It was not until 2007 that our bureau further improved the measures for voltage equalization, shielding, and shunt of weighing sensors, hired lightning arrester manufacturers to produce specialized lightning arresters for weighing sensors, and implemented installation that the lightning protection safety problem of electronic weighing scales was basically solved. From this, it can be seen that the comprehensive lightning protection of electronic weighbridges, especially the protection of weighing sensors, is the most difficult, and is also a key factor in the design and construction of electronic weighbridge comprehensive lightning protection engineering.

2、Load CellAnalysis of the Composition and Characteristics of Lightning Vulnerability

2.1 Composition of weighing sensor: The weighing sensor is the core component of the electronic weighbridge, which is a functional converter that converts pressure into electricity. It is mainly composed of an elastic body, a resistance strain gauge, and a detection circuit. The weighing sensor is actually a load-bearing detection instrument, with circuits including bridge measurement circuits, LC filtering circuits, A/D circuits, and microcontroller circuits. The function of the weighing sensor is to complete the weighing and weighing process of the electronic weighbridge with the cooperation of the junction box and secondary instruments.

2.2 Analysis of the characteristic characteristics of load cells susceptible to lightning strikes

2.2.1 Special assembly and usage environment conditions: The weighing sensor is assembled around the lower edge of the large metal scale platform (the electronic weighbridge is composed of several weighing sensors); Due to the need to bear gravity, the elastic body, strain gauge, and shell of the weighing sensor are all made of special metal, and it is difficult to achieve insulation between the elastic body and the weighing platform; Electronic weighing scales, also known as car scales, are often installed outdoors due to traffic restrictions, making them more susceptible to lightning strikes compared to other equipment.

2.2.2. Poor resistance to overvoltage and overcurrent: The weighing sensor chip belongs to microelectronic products, and usually the working voltage of weak current equipment is only 5-12V, with a working current not exceeding 20MA. Induced lightning strikes can cause equipment damage.

2.2.3. Low insulation impedance: The withstand voltage limit between the elastic body of the weighing sensor and the electronic circuit is only 1KV to 1.5KV. The high voltage induced on the elastic body of the weighing sensor will cause breakdown of the strain gauge of the weighing sensor and the corresponding circuit behind it.

2.2.4. Long exposed transmission signal cables: The power and signal of the sensor are transmitted through cables, while the weighing sensor is installed outdoors, which inevitably requires a longer transmission cable. Lightning strikes can easily generate high potential on the transmission cable through electromagnetic induction.

3. Technical Measures for Lightning Protection of Weighing Sensors

The lightning protection of the weighing sensor isElectronic WeighbridgeThe key to comprehensive lightning protection is that this article does not cheat on the conventional lightning protection measures for electronic weighing scales (including lightning protection, grounding, power protection, etc.), and focuses on the technical measures for voltage equalization, shielding, and shunt lightning protection at the weighing sensor position.

3.1. Equalizing voltage: Equalizing voltage refers to making an equalizing ring at a suitable position on the base of the weighing sensor, connecting the conductors near the weighing sensor at ground potential, and connecting the equalizing ring to the grounding device. When strong lightning passes through, it ensures that there is no harmful potential difference between conductive components, no side flashover discharge, and no damage to the weighing sensor. The specific approach is to ensure good electrical connection between the weighing platform, weighing sensor base, transmission cable shielding steel pipe, and junction box and grading ring.

3.2 Shielding: Shielding is the most effective method to intercept the intrusion of lightning electromagnetic pulse waves into the weighing sensor channel. The specific method is to lay all signal transmission cables from the weighing sensor to the junction box through steel pipes and ensure proper grounding.

3.3. Shunt: Shunt refers to the installation of an appropriate lightning arrester between the equipment to be protected and the lightning protection grounding wire. When direct lightning strikes or lightning effects generate overvoltage waves on the line and enter the equipment along these wires, the resistance of the lightning arrester will suddenly drop to a low value, diverting the lightning current into the ground. There are two points to pay attention to in the diversion of symmetrical weight sensors. Firstly, the weighing sensor is connected to the junction box through a signal transmission cable, so lightning arresters should be installed at both ends of the signal transmission cable. Secondly, it is necessary to design and manufacture a dedicated lightning arrester that matches the weighing sensor, ensuring that the main technical parameters are: the limiting voltage of the lightning arrester is less than 12 volts; The reaction action time is less than 1 ns; The attenuation of the signal after connection is between 0.1dB and 0.5dB; The maximum current capacity for lightning protection is 5KA.

4. Conclusion

By taking measures to equalize, shield, and divert the voltage of the weighing sensor, as well as conventional lightning protection measures for electronic weighing scales, the lightning protection safety of electronic weighing scales can be basically achieved.